1. Field of the Invention
The present invention relates to a light scanning apparatus used in an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunctional peripheral thereof.
2. Description of the Related Art
As a light scanning apparatus used in an electrophotographic image forming apparatus, a light scanning apparatus having the following configuration is well known. Specifically, there has been known a light scanning apparatus configured to deflect a light beam emitted from a light source by a rotary polygon mirror and to guide the deflected light beam onto a photosensitive surface of the photosensitive member by optical components such as a lens and a mirror, to form a latent image on a photosensitive member. An imaging optical system is formed of at least one fθ lens. The fθ lens has a special lens effective surface typified by an aspheric surface for the purpose of enhancing scanning characteristics. Further, a housing (hereinafter referred to as “optical box”) configured to support and fix members of an optical system is formed of a resin molding because of the advantages such as the securement of a degree of freedom of a shape, the reduction in weight, and the reduction in price. In particular, the use of an optical box made of a resin is greatly advantageous in a tandem type image forming apparatus, because a large number of optical components are used, and further mounting directions and mounting methods of supporting and fixing the optical components are not uniform. On the other hand, the optical box made of a resin has a large coefficient of thermal expansion under the condition of an increased temperature, compared to an optical box made of a metal. Further, the optical box made of a resin has a low coefficient of thermal conductivity, compared to the optical box made of a metal. Therefore, when the optical box made of a resin is used, in the light scanning apparatus containing a heat source, a temperature distribution becomes non-uniform, and a hot portion and a cold portion occur partially. As a result, warpage and local distortion in different directions occur in the optical box.
A deflection member such as a rotary polygon mirror having a plurality of deflection reflecting surfaces has been often used in the light scanning apparatus. When the deflection member is driven, the temperature increases by the heat generation from a driving portion such as a motor. If light scanning is performed continuously for a long period of time, a rotation shaft receiving portion of the rotary polygon mirror and an IC chip mounted on a motor portion configured to drive the rotary polygon mirror are heated to a high temperature. Further, even in the case where light scanning is performed for a short period of time, the temperature logarithmically fluctuates and increases immediately after the start of the rotation of the motor. Therefore, the optical box is greatly distorted and deformed by a non-uniform increase in temperature in the light scanning apparatus. A lens, the rotary polygon mirror, and a mirror constituting the light scanning apparatus are housed in the optical box, and hence the positions of optical components such as the lens and the mirror are changed by the deformation of the optical box, with the result that the path through which a light beam passes and a reflecting direction are changed with time.
The deformation of the optical box occurs mainly for the following two causes. The first cause is a hot current of air, and the second cause is a radiation heat. The hot current of air which is the first cause occurs as follows. The rotary polygon mirror rotates at a high speed and hence generates wind, which absorbs heat generated on the periphery of a deflector to become hot current of air. The periphery of the deflector is surrounded by a rib provided perpendicularly to a bottom wall of the optical box so as to keep the strength of the optical box, and the way of the hot current is blocked by the rib so that the hot current of air stagnates around the deflector. Because of this, an increase in temperature on the periphery of the deflector becomes larger than that in the other portions of the optical box. Consequently, the optical box increases in temperature locally, which causes the deformation such as warpage and distortion in the optical box. Further, the radiation heat which is the second cause occurs as follows. The rotary polygon mirror rotates at a high speed, and hence the rotation shaft receiving portion of the rotary polygon mirror fitted in the optical box so as to position the deflector reaches a high temperature. Further, the IC chip mounted on the motor portion also generates heat. Then, portions immediately below the rotation shaft receiving portion and the location where the IC chip is mounted are heated locally by the radiation heat, with the result that the optical box expands and deforms.
As described above, by the deformation of the optical box, directions and amounts of light beams vary in different color stations so that light-condensing positions on surfaces to be scanned are changed, and thus a horizontal direction, a vertical direction, or a magnification of an image line is fluctuated, resulting in the degradation in image. In particular, in a tandem type image forming apparatus, a light beam position of each color is fluctuated, and hence color misregistration occurs when toner images of respective colors are superimposed. In order to solve the foregoing problem, there has been proposed a light scanning apparatus in which a rib, which has provided perpendicularly to the bottom portion of a housing main body, is inclined so that the hot current of air generated from a rotary polygon mirror and so on is diffused along the inclined rib (Japanese Patent No. 4170736).
However, according to the conventional method, certain effects are obtained with respect to the first cause of the deformation of the optical box described above, but there is a risk in that effects may not be obtained with respect to the second cause. That is, hot current of air generated from the deflector can be diffused to the periphery; however, hot current of air is blown to the inclined portion of the optical box, with the result that the entire optical box is greatly distorted by the distortion caused by the expansion and deformation of the inclined portion. The inclined portion of the optical box has a straight line shape, and hence the expansion thereof serves to distort the entire optical box. Further, the local deformation of the rotation shaft receiving portion of the rotary polygon mirror and also the deformation of expansion from the center of the rotation shaft receiving portion to the periphery thereof cannot be absorbed by the inclined portion formed into a straight line shape, which leads to the deformation of the entire optical box.